A rate increase on the price of First-Class Mail, or Forever, stamps went into effect on Sunday, according to an United States Postal Service news release.

The new cost of a Forever stamp is 55 cents, a five-cent increase, the USPS said in the release.

The 10 percent rise in cost from the previous price of 50 cents is the largest increase from the USPS since 1991, when the rate spiked 16 percent as the cost of a stamp went from 25 to 29 cents, USA Today reported.

A Star Trek-themed Forever stamp Thursday Jan. 24, 2019. The price of first class postage for a letter will increase Sunday to 55 cents. Forever stamps purchased before then will continue to be honored at the new rate.(Photo11: Ronald W. Erdrich/Reporter-News)

But the 5-cent increase is the largest monetary jump in U.S. history, according to The Hill.

Forever stamps purchased prior to Jan. 27, 2019 will live up to their name and remain “valid for mailing a letter regardless of how much the price changes,” per NBC News.

The USPS is raising rates on other products too.

All flat rate boxes and envelopes, including priority mail, increased in cost, as well as metered mail rates, according to the USPS news statement.

Outbound international letters and domestic postcards remain the same price, while additional ounces on a first-class letter is cheaper, dropping from 21 to 15 cents, per the USPS news release.

Unlike many other federal agencies, taxes do not fund the USPS, which “runs on the sale of its products and services,” USA Today reported.

In 2018, the USPS reported it lost $3.9 billion, according to The Hill.